Functionality of recycled concrete fines

Abstract

The pressing challenges of climate change and the increasing surplus of concrete waste is making it increasingly important to fully recycle concrete in a sustainable way. Since cement is by far the component that contributes most to the environmental footprint of concrete, it is of great importance to investigate the potential of upcycling secondary cementitious materials. Modern recycling techniques such as the Smart Liberator make it possible to efficiently separate the coarse and fine fractions in concrete waste. It also makes is possible to produce a fine powder fraction that shows potential to replace primary cement, which helps to mitigate CO2 emissions. This study explores to which extent recycled concrete fines (RCF), obtained from advanced recycling technologies such as the Smart Liberator, can replace ordinary Portland cement (OPC) in mortar mixtures. It is assumed that the RCF consist mostly of cementitious materials. The aim of this study is to assess the performance of RCF when replacing OPC in different mortar mixtures. Several treatment methods were applied to enhance the reactivity of the RCF.
To be able to assess the performance of the RCF as replacement of ordinary Portland cement at mortar level, three reference mortar mixtures, made from CEM I, CEM II/B-V and CEM III/B, were prepared for comparison. Portland cement with strength class 52.5 R was used in for preparation of the reference mixtures. The used RCF were the 0-63 μm fraction obtained from the Smart Liberator. It is expected that the RCF are from CEM III origin based on documentation from the company Rutte Groep. However, due to limited stock, some of the mortar mixtures had to be prepared with a different type of RCF, consisting of RCF with a CEM I and CEM III origin that were mixed in a 1:1 ratio. The OPC in the reference mixtures was replaced with RCF at 50%, 75% and 100% ratios.
Grinding, carbonation, conventional heating and flash calcination were applied as treatments to enhance the reactivity of the RCF. Thermogravimetric analyses were used to assess the effects of the treatments on the decomposition of the compounds of the RCF. Mortar prisms made with treated and untreated RCF were tested for flexural and compressive strength after 28 days of curing, and their performance was compared to the reference mortars.
The results showed that the untreated RCF performed best when used in combination with blast furnace slag. This combination was the only one in which a 100% replacement resulted in measurable strengths and the measured strengths at the other replacement ratios were the highest, reaching up to 70% of the compressive strength of the reference prisms at 50% replacement. This indicates that the RCF are a suitable activator for blast furnace slag.
The effects of the treatments differed in the different mixtures and also varied across the replacement ratios. Grinding of the RCF improved their performance at the 50% and 75% replacement ratios in the CEM I variant, while their performance in the CEM II and CEM III variants was similar to or lower than the untreated RCF. The same conclusions are drawn from the results after carbonation. Heating of the RCF up to 900 °C for 30 minutes in a regular chamber furnace resulted in better results for the CEM I variants at all replacement ratios, while no significant improvements were measured for the CEM II variants. For the CEM III variants, only the 75% replacement ratio had a significant improvement. Lastly, flash calcination resulted in the most significant improvements for the CEM I and CEM II variants at all replacement ratios. For the CEM III variants, flash calcination significantly improved the strengths at the 100% and 75% replacement ratio, but not at the 50% replacement ratio. Since this does not align with each other, it is necessary to reconfirm these results.
As a conclusion, this study demonstrated that untreated RCF shows potential to BFS, while flash calcination is the most promising treatment for boosting RCF reactivity across most mixtures. Conventional heating also showed potential, especially for CEM I. However, grinding and carbonation had mixed results, with limited benefits for BFS combinations due to carbonation effects. Future research should confirm these findings and explore ways to optimize the treatment methods.